The application of sintered NdFeB permanent magnets mainly relies on their magnetic properties. However, due to different usage environments and conditions, in addition to magnetic performance requirements, there are also mechanical and chemical performance requirements for the magnets. For example, some permanent magnets are used in high-speed rotating machinery and must withstand large centrifugal forces, are used in vibration environments, withstand extremely high acceleration (3g~5g), or are subject to stress when installing the magnets. It may peel, fall off, corners or crack, etc., so it is natural to have requirements for the mechanical properties of the magnet.
What are the Mechanical Properties of Materials?
The mechanical properties of materials generally include strength, hardness, plasticity, and toughness. These mechanical property parameters have different physical meanings.
Strength refers to the maximum ability of a material to resist the damaging effects of external forces. Intensity is divided into different types according to the different forms of external force:
- Tensile strength (tensile strength) refers to the strength limit when the external force is pulling force
- Compressive strength refers to the strength limit when an external force is pressure
- Bending strength refers to the strength limit when an external force is perpendicular to the axis of the material and causes the material to bend.
Hardness refers to the ability of a material to locally resist hard objects from being pressed into its surface. It is an indicator for comparing the softness and hardness of various materials. The higher the hardness, the stronger the metal's ability to resist plastic deformation.
Plasticity refers to the ability of a solid material to resist deformation under a certain external force. A material can permanently deform without being destroyed under the action of external force.
Toughness represents the ability of a material to absorb energy during plastic deformation and rupture. The better the toughness, the less likely a brittle fracture will occur. In materials science and metallurgy, toughness refers to the resistance of a material to breaking when it is subjected to a force that causes it to deform. It is the ratio of the energy that the material can absorb before breaking to its volume.
Mechanical Properties of Sintered NdFeB
Sintered NdFeB is a brittle material. Its mechanical properties are hard and brittle, that is, high strength and low toughness. There is almost no plastic deformation before fracture, that is, it fractures during the elastic deformation stage.
The figure below is a comparison of the magnetic energy product (BH)m and fracture toughness of various permanent magnet materials. We can find that sintered NdFeB has the highest magnetic energy product (BH)m, and the fracture toughness is still comparable to Sm2Tm17, SmCo5, and ferrite permanent magnets, because they are all permanent magnet materials based on intermetallic compounds and belong to Brittle materials. Bonded rare earth permanent magnet materials, Fe-Cr-Co, and magnetic steel have the best fracture toughness, but their magnetic energy product (BH)m is much lower than that of sintered NdFeB.
Brittle materials commonly use three indicators to describe the mechanical properties of the material:
1. Fracture toughness usually reflects the strength when cracks expand in the material. The unit is MPa·m1/2. The fracture toughness test of materials requires a tensile testing machine, stress sensor, extensometer, signal amplification dynamic strain gauge, etc. In addition, the sample must be made into a thin sheet.
2. Impact strength (impact fracture toughness) reflects the energy absorbed by the material during the fracture process under the action of impact stress. The unit is J/m2. The measured value of impact strength is too sensitive to the size, shape, processing accuracy, and test environment of the sample, and the dispersion of the measured value will be relatively large.
3. The flexural strength is measured by the three-point bending method. Because the sample is easy to process and the measurement is simple, it is most commonly used to describe the mechanical properties of sintered NdFeB magnets.
The high strength and low toughness characteristics of sintered NdFeB permanent magnet materials are determined by their crystal structure. In addition, the following two factors will affect the bending strength of sintered NdFeB permanent magnets and are also ways to improve their strength.
1. The Nd content has a certain influence on the strength of sintered NdFeB. Experimental results show that under certain conditions, the higher the Nd content, the higher the strength of the material.
2. Adding other metal elements has a certain impact on the strength of sintered NdFeB. When a certain amount of Ti, Nb, or Cu is added, the impact fracture toughness of the permanent magnet is improved; when a small amount of Co is added, the flexural strength of the permanent magnet is improved.
The comprehensive mechanical properties of sintered NdFeB are not high enough, which is one of the important reasons limiting its application in wider areas. If the toughness of the product can be improved while ensuring that the magnetic properties are improved or unchanged, it will make the sintered NdFeB in Military, aerospace, and other fields play a greater role and enter a new period of development.